When we left them, Marty, the Doc and Jennifer were heading for the future. "It's your kids, Marty," the Doc was saying. "Something's got to be done about your kids!" Marty looked troubled. He didn't have any kids. Yet. Soon he finds out. It's a shock.

In the future, Marty's son is about to be arrested for a crime, organised by Griff, grandson of Biff, his old enemy. The Doc, who has access to news coverage, attempts to alter the facts, with Marty's help.

But there's worse to come. Marty discovers that his son's a wimp, the family are living in the worst part of town and his middle-aged self is about to lose his job for unprofessional behaviour. When Jennifer meets 48-year-old Jennifer, she collapses and has to be rescued and brought back to the Eighties.

If that sounds complicated, you ain't seen nothin' yet. The DeLorean time car malfunctions and brings them down into an altered reality. The "new", anarchic, Beirut-style Hill Valley, 1985, is in the hands of Biff, who has become the richest man in America, now married to Marty's mom, an alcoholic, after his dad's murder in '73.

It appears that the entire turn of events depended upon something Marty did just after arriving in 2015, at the start of their adventure. The Doc calculates a way of changing history again by going back to 1955 on the day of the Enchantment Under The Sea dance, the night of the thunderstorm, when Marty saved his own birth.

Robert Zemeckis uses special effects to advance the story, which rushes at such a pace it's hard to catch your breath. All the humour, excitement and pleasure of the first film are intact. It's faster, less perfectly formed, more inventive and still running.